How big is the Question. would it be better to use the Metal router Bit or would it be easer to get a Hole cutter for a drill that you can vary the size of? Personaly if it is a 1 inch hole or bigger go w/ a Hole cutter bit for a drill If it is a Breather tube hole go w/ the dremmal.
Anything over 1/2" calls for a hole saw..Yea you can whittle out a 3" diam hole with a dremel and a burr, or rat tail file, but a hole saw is the way to go for larger holes.